What do you do after starring in one of the most successful British films of all time? If you are Ewan McGregor, you grab your light sabre and sign a long-term deal for Star Wars. If you are Bobby Carlyle you take a turn as a James Bond villain. And Ewen Bremner? If you're the man who as Spud mainlined into the Trainspotting audience's funny bone, you take roles in a number of low-budget, little-seen flops, before opting for art-house kudos with American wunderkind Harmony Korine, and mainstream glamour with the forthcoming blockbuster Pearl Harbor.

Unusually for a film star, Bremner is better looking in real life than on screen. Still in his 20s, fresh-faced, with hair closer to strawberry blond than ginger, his features are drawn in bold, angular lines. Bremner made his film debut as a dim-witted schoolboy in the comedy Heavenly Pursuits, 10 years before Trainspotting. With a resumé ranging from Shakespeare to Judge Dredd, he has also produced a string of unforgettable cameo appearances, most recently in Snatch, when Vinnie Jones shuts the car window on his head and drives off.

Bremner first met writer-director Harmony Korine several years ago. Korine had shocked public opinion with Kids, a portrait of underage sex and drug use which he wrote in his teens. Bremner had been "blown away" by Korine's directorial debut Gummo, a low-budget drama about white-trash Ohio kids who kill cats and sell the corpses to a supermarket.

"I'm making a movie that's going to be like nothing else," was Korine's modest pitch for Julien Donkey-Boy, the "plot" of which was inspired by his Uncle Eddie, who had been in a mental institution for years. The finished script was simply a description of a series of scenes. The actors would mix with members of the public, improvising their lines and being recorded on tiny "spy cameras".

Korine had been impressed by Bremner's performance as Archie, the inarticulate, twitchy Scots headbanger in Naked, and he suggested Bremner play the supporting role of Julien's brother, an aspiring wrestler and misfit in a film entirely populated by misfits. "I said, 'I want to play Julien,'" Bremner recalls, "and I think he was delighted that I had the audacity to say that I wanted to do it."

Initially, however, Bremner had grave reservations about his ability to pull off the American accent that would be required. "I believed that I would be capable of it, but I'm not an actor who is gifted with that aptitude for accents. It was a real struggle for me to lose my Scottish accent. That was the hardest part, it wasn't the psychological aspects of Julien." Bremner tried out his accent when he went to visit Korine's Uncle Eddie. "It was quite difficult to get access to him, but they sneaked me in as a relative." He listened to Eddie's fantastical stories, studied his speech patterns - "so fast and so slurred" - and crafted a character. He maintained his accent off-set and refused to see Scots friends, for fear those Rs would start rocking and rolling again. "My girlfriend was losing her patience with me when I was on the telephone... I didn't think I was in character, I was just trying to be American." Bremner also totally changed his appearance for Julien and is almost unrecognisable with black, loosely-permed hair and metal teeth.

Julien Donkey-Boy is unlike any other film, not just in content, but in tone and ambiguity. In the opening scene, Julien appears to attack and possibly kill a boy at a pond. The incident is never referred to again. Julien works as an assistant at a blind school and is part of a dysfunctional family that includes Chloe Sevigny as his pregnant sister, and the German director Werner Herzog as the father who forces Julien to slap himself for being stupid, and hoses down his other son with cold water because it is character-forming.

The film is composed of seemingly dislocated scenes, including a nun masturbating and spanking herself, but a loose storyline eventually starts to emerge, and there is considerable black comedy. "Stop that shivering," says the father, aiming the hose at his son. "A winner doesn't shiver." Meanwhile Julien asks passersby which tree is his family tree.

The San Francisco Chronicle called Julien Donkey-Boy a "self-indulgent mess", while Entertainment Weekly compared it to the poetry of Robert Frost. "It's probably the thing that I feel most proud of," says Bremner, who, in Julien, has created a character both touching and appalling. He admits it was a huge relief when it was over. "I had been living in a different world," he says. "Personally I had really isolated myself to do that job. It took me a couple of weeks or so to get it out of my system."

The film left Bremner not only physically and mentally exhausted, but skint, having spent five months working for next to nothing. It did, however, get him an American agent and a reputation as a Brit who can do an American accent - which ultimately led him to director Michael Bay, producer Jerry Bruckheimer, and their take on the Japanese attack on Pearl Harbor. Bremner doubts the makers of The Rock and Armageddon saw Julien Donkey-Boy. "I'm sure it's not a film that they would want to watch," he says.

Going from a no-budget, straight-to-video film to a $145m movie must have been quite a culture shock. "I don't see Julien as a product," says Bremner. "But I see this as a product, a very custom-built blockbuster, a classic of the genre. I don't always have hundreds of options and this was a pretty good one."

In Pearl Harbor Bremner plays Affleck's co-pilot, but he laughs at the notion that they might replace Cruise and Kilmer as the top guns for the 21st Century. "There's a lot of stunts and special effects," he says. "We're running around firing guns and that sort of thing, but I'm not putting myself in much danger."

He did have to go to boot camp, however. "They really broke us down, me particuarly. The last night of boot camp they were sending someone in to check on me every few minutes, to check I was still breathing." He maintains his character is not so different from his previous hapless supporting characters. "I play an American pilot with a speech impediment, who falls in love and goes to war and becomes a man or a hero. I'm just making Ben Affleck look good, ha-ha."

 Julien Donkey-Boy opens on Friday. Pearl Harbor will be released next year.